Corrosive liquids, such as strong acids and bases, and highly reactive chemical solutions, are frequently used in chemical processes. Liquids may also be erosive, due to, for example, suspended solids in the liquid. Valves are a particularly vulnerable to both corrosive and erosive environments due to the necessity or maintaining the integrity of the fluid seal and the mechanical parts of the valve. Valves have frequently been lined with various types of materials such as rubber, glass, plastic and the like to combat either a corrosive or erosive environment. For example, to provide corrosion resistance iron valves may be lined with glass, or various rubbers such as butyl rubber, hard rubber, natural rubber, neoprene and the like. For erosion resistance, iron valves may be lined with hardened steel, or the like. Also, valves may be lined with various kinds of plastics, such as TEFLON.TM. and fiberglass-reinforced plastic. Lead has also been used to line valves, and many valves are coated with a thin layer of glass, which is generally applied in the form of low-temperature melting frit and then heated to form the glass lining in place.
while many lining materials are suitable for corrosive or erosive environments, most are not suitable for environments which are both erosive and corrosive, such as liquid acids containing abrasive solids. Metal valves lined with rubber, glass, lead and the like are commercially used in environments which are both corrosive and erosive, but the lifetime of these valves is relatively short in these environments, particularly where the fluid stream is moving at high velocities. None of these lining materials have a long life in handling a high-velocity, corrosive, and erosive liquids.
Ceramics are generally known for both corrosion and erosion resistance. Valves made of a ceramic material, however, can be difficult to form and consequently are relatively expensive. Furthermore, while ceramic devices can be corrosion and erosion resistant, they generally possess poor flexural stress resistance and impact toughness, so that a valve formed completely of ceramic could be easily fractured.
U.S. Pat. No. 3,349,795 to Matsutani discloses a diaphragm valve with a complex ceramic valve casing. The ceramic valve casing is a complex shape with an arcuate top surface formed across the fluid path to form a weir curving downwards to a circular opening. The complex shape of Matsutani ceramic casing limits the process of manufacture for the shape mostly to slip casting methods. Such a complex shape is not conducive to fabrication from high-purity, high density ceramics materials, which on a commercial scale require fabrication by powder compaction methods, particular isostatic and uniaxial compaction methods. In addition, common machining methods, such as lathe, mill, and cylindrical grinding, and surface grinding, cannot be used to form the shape. The ceramic materials which can be made into the shape of a Matsutani casing by slip casting methods are usually of a porcelain variety and typically of low purity, and do not have the corrosion resistance, particularly to highly caustic solutions, for many environments. In addition, the complex shape of the Matsutani ceramic casing requires a specially constructed two-piece valve body in order to insert the casing into the body.
An additional problem with complex ceramic shapes, such as in the Matsutani reference, is that flexural stresses are inevitably induced in the ceramic shape which cause these shapes to be quite fragile. In addition, such one piece complex shape is subject to thermal flexural stresses due the differing thermal expansion coefficients of the ceramic and the metal of the valve material.